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1. Field of the Invention
The invention relates generally to the field of geophysical mapping of subsurface Earth structures. More specifically, the invention relates to systems and methods for mapping changes in content or composition of Earth formations over time.
2. Background Art
Geophysical mapping techniques for determining subsurface structures in the Earth include, for example, seismic surveying, magnetotelluric surveying and controlled source electromagnetic surveying, among others. In seismic surveying, an array of seismic sensors is deployed at the Earth""s surface (or near the water surface or on the water bottom for various types of marine seismic surveying), and one or more seismic energy sources is actuated at or near the Earth""s surface in a location near the seismic sensor array. A record is made, indexed with respect to time of actuation of the seismic energy source, of signals corresponding to seismic energy detected by each of the sensors in the array. Seismic energy travels downwardly from the source and is reflected from acoustic impedance boundaries below the Earth""s surface. The reflected energy is detected By the sensors. Various techniques are known in the art for determining the structure of the subsurface Earth formations below and/or adjacent to the sensor array from recordings of the signals corresponding to the reflected seismic energy. Other techniques known in the art provide estimates of fluid content in porous Earth formations from characteristics of the reflected energy such as its phase and/or amplitude.
A particular technique for seismic mapping includes resurveying substantially the same subsurface area of the Earth at selected times after the initial seismic survey is performed. One purpose of such repeated seismic surveying is to determine the extent to which fluids in the pore spaces of permeable Earth formations have moved. A particular application for mapping fluid movement is to determined changes in subsurface reservoir content as economically useful fluids, such as petroleum, are produced from such permeable formations. Such repeated seismic surveying is known in the art as four dimensional (4D) seismic surveying. Using 4D seismic, it is possible, for example, to determine where oil and/or gas have been withdrawn from a petroleum-bearing permeable formation (xe2x80x9creservoirxe2x80x9d) and have been replaced by water. Such determination of oil, gas and water movement is particularly useful in determining whether water may unexpectedly be produced from a particular wellbore that penetrates the reservoir. 4D seismic surveying may also be used to determine prospective locations and/or geologic targets for wellbores to be drilled through the Earth to produce oil and/or gas which account for the movement of oil and/or gas from their originally determined locations in a reservoir.
4D seismic has the advantage of being relatively easily performed, and may enable mapping of subsurface fluid movement without the need to penetrate reservoirs in a large number of spaced apart locations.
Determining movement of fluids within Earth formations using 4D seismic techniques, however, requires that the fluids to be monitored cause the formations in which they are disposed to undergo a detectable change in seismic properties as the fluid content changes over time. In some cases, such as where oil and water in a reservoir have similar acoustic properties, it may be difficult to monitor oil and water movement in a reservoir using 4D seismic.
Another subsurface structure determination method known in the art is magnetotelluric (MT) surveying. MT surveying is described, for example in K. Vozoff, The Magnetotelluric Method in the Exploration of Sedimentary Basins, Geophysics 37, 98-141 (1972). Generally speaking, the MT method of subsurface structure mapping includes deploying an array of electric field and magnetic field sensors at the Earth""s surface. Electromagnetic fields are induced in the Earth by ion currents moving in the Earth""s ionosphere. The ionospheric currents induce substantially planar electromagnetic waves that radiate downwardly, and into the Earth. The array of sensors detects electric and magnetic fields induced by the plane wave in the Earth formations. The magnitude of the plane wave-induced electromagnetic fields at any position along the Earth""s surface is related to the spatial distribution of electrically conductive materials in the Earth. MT methods of mapping have the advantages of using relatively inexpensive, easy to deploy sensors, and not needing a separate energy source to activate the Earth formations. MT techniques, however, require that the structures which are mapped be sufficiently electrically conductive to produce detectable electromagnetic field components at the Earth""s surface. Petroleum-bearing formations, for example, are electrically resistive, as compared with the surrounding Earth formations. Mapping movement of petroleum (oil or gas) using MT techniques alone, therefore, has proven to be difficult.
Other systems and techniques for monitoring movement of fluid in Earth formations include permanently-emplaced sensors disposed in selected wellbores drilled through the Earth. Such sensors may include electrical resistivity sensors, natural radiation detection devices, acoustic sensors and other types of sensing devices known in the art for monitoring movement of fluids. See, for example, U.S. Pat. No. 5,886,255 issued to Aronstam. Using techniques such as disclosed in the Aronstam ""255 patent can be expensive and, relatively speaking, somewhat unreliable because of the number of sensor systems which are used in such techniques. Other systems for reservoir monitoring and/or production control using permanently emplaced sensors are described, for example in U.S. Pat. No. 5,597,042 issued to Tubel et al., and U.S. Pat. No. 5,662,165 issued to Tubel et al. Geophysical systems for down hole measurements are disclosed by Vinegar et al. in published U.S. patent application Ser. No. 2002/0043369 A1 and include mostly temperature, pressure and acoustic sensors.
Other techniques for monitoring movement of petroleum in reservoirs include xe2x80x9cwell loggingxe2x80x9d at selected times using pulsed neutron (neutron capture cross section) instruments such as one sold under the trade name PDK-100 by Baker Hughes, Inc., Houston, Tex. Well logging includes lowering a measuring instrument into the wellbore at the end of a drill pipe, coiled tubing, or most commonly, at the end of an armored electrical cable. As the instrument is moved into or out of the wellbore, a record is made with respect to depth of the measurements made by the instrument. Such well logging techniques provide a determination within each wellbore surveyed of a depth of a hydrocarbon/water contact depth. Over time, as a reservoir produces oil and/or gas, the depth of the contact in each wellbore that penetrates the reservoir may change. By determining the depth in a number of wellbores at selected times, a change in the distribution of the contact over time may be determined. It is difficult and expensive to log individual wellbores, particularly when the wellbore is producing, because the petroleum production from the wellbore must be stopped (xe2x80x9cshut inxe2x80x9d) during well logging operations Shutting in and logging a large number of wellbores to determine changes in the distribution of the contact can be difficult and expensive using well logging techniques known in the art. Further, some reservoirs may not have a sufficient number of wellbores that penetrate the reservoir in order to accurately map changes in distribution of the contact.
Methods to map the conductive parts of the formation are disclosed by Torres-Verdin et al. in U.S. Pat. No. 5,767,680 in which AC and DC electrical measurements are used to define the shape and location of oil water interfaces.
Other methods such as are disclosed in U.S. Pat. No. 6,266,619 B1 include data mining of the subsurface and matching to production history to optimize well control.
What is needed, therefore, is a system for mapping changes in fluid content of Earth formations which can be used where there is little acoustic impedance contrast between moved fluids, where the moved fluids are relatively electrically resistive, and which does not require permanently emplaced sensors in substantially every wellbore drilled through a reservoir.
One aspect of the invention is a method for monitoring a reservoir. The method includes making a first set of electromagnetic measurements at selected locations along the Earth""s surface, and making a first measurement from at least one sensor disposed proximate the reservoir in a wellbore. An initial Earth model is determined from the first electromagnetic and first sensor measurements. The initial Earth model includes a fluid contact. At selected times, the sensor and electromagnetic measurements are repeated and a spatial distribution of the fluid contact is determined from the repeated measurements.
Another aspect of the invention is a system for mapping structures within the Earth. A system according to this aspect of the invention includes a plurality of electromagnetic sensors disposed in a selected pattern on the Earth""s surface, at least one sensor disposed in a wellbore drilled proximate a subsurface structure to be mapped, and a means for mapping the subsurface structure from measurements made by the electromagnetic sensors and the at least one sensor. In some embodiments, the electromagnetic sensors include magnetotelluric sensors. In some embodiments, the electromagnetic sensors include controlled source electromagnetic induction sensors.
Another aspect of the invention is a method for monitoring a reservoir including making a first set of galvanic measurements at locations along the Earth""s surface. A first set of measurements is made from at least one sensor disposed proximate the reservoir in a wellbore. An initial Earth model is determined from the first galvanic and first sensor measurements. The initial Earth model includes a spatial distribution of a fluid contact. The method according to this aspect includes repeating the sensor measurements and galvanic measurements at selected times and repeating determining the spatial distribution of the fluid contact from the repeated measurements. Some embodiments of a method according to this aspect of the invention include making electromagnetic measurements such as induction or magnetotelluric measurements. Determining the initial Earth model, and determining spatial distribution of the fluid contact takes account of the electromagnetic measurements.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.